This Invention relates to radio location systems. A number of systems are being developed for identifying the location of a mobile unit, using radio propagation characteristics. One such system is the Global Positioning System (GPS), in which a portable unit obtains a position fix using radio transmissions from space satellites. This system is highly accurate, but requires special equipment, and is unreliable in locations having poor visibility of the sky, because several widely separated satellites must be in line-of-sight relationship with the handset for a fix to be obtained.
Several proposals have been made for systems which use the radio propagation characteristics of a cellular radio system to provide a position fix for a cellular radio mobile unit. This would allow the mobile unit itself to act as a position finding device. As is well known, cellular radio systems allow a user having a portable handset (a "mobile unit") to make and receive telephone calls, either to another mobile unit or to a conventional fixed termination, by means of a radio link. The radio link is established between the mobile unit and one of a network of fixed radio base stations distributed over the area to be covered. The system allows any mobile unit to communicate through any of the base stations; usually the mobile unit will communicate through the base station providing the best quality radio signal.
Because the mobile unit may move during the course of a call, it can become necessary for it to move out of range of the base station with which the call was initially established. Cellular radio systems therefore include handover systems to allow communication to be established with a second base station, and dropped from the first, without interrupting the call itself as perceived by either party to the call. In the system known as GSM, (Global System for Mobile communications), the mobile unit frequently monitors the BCCHs (Broadcast Control CHannels) of the surrounding base stations in order to establish which base station is providing the best signal, and therefore through which base station a new call should be established, or whether a handover should be initiated. This process occurs in both idle and active modes, i.e. there is no need for the user to make a call.
Developments in GPS technology mean that a highly accurate synchronisation source can now be implemented relatively cheaply at each cellular radio base site. A good source of synchronisation has a number of benefits, these include; improved handover, an ability to reduce the effect of interference between neighbouring base stations, and enabling highly accurate radiating frequencies on the radio interface. It should be noted that unlike simple broadcast time signals, the GPS synchronisation signal takes the position of the GPS receiver into account, and can therefore compensate for the time lag caused by the finite speed of radio waves.
European patent Specification EP0320913, (Nokia), describes a system in which timing pulses derived from the GPS system are transmitted from each of three or more base stations, and their different arrival times at the mobile unit are used to identify the position of the unit. This prior art system requires the mobile unit to interrogate each base station in turn, which requires it to hand over communication between the various base stations in order to carry out this interrogation. This requires the use of several traffic channels, or an auxiliary channel and also requires that reliable radio communication can be established with each nearby base station.
In International Patent Application WO95/00821 (Omniplex) each base station transmits synchronised packet data signals. The mobile unit monitors all the base stations' packet data channels simultaneously, which either requires a mobile unit capable of receiving several radio frequencies at once, or that all the base stations transmit their data packets on the same channel. Neither of these features are conventional in a cellular radio system.
Both of these systems also require the transmission of special timing or synchronisation pulses from the cell sites (base stations) to the mobile unit, and the recognition of these pulses by the mobile unit. This requirement not only imposes a signalling overhead on the mobile unit, but it requires additional functionality in the mobile unit to recognise the timing pulses.